1. Field of the Invention
The present invention pertains to MOS devices suitable for VLSI stuctures. More particularly, the present invention pertains to high-frequency MOS devices.
2. Discussion of Related Art
CMOS is advantageous for VLSI circuits, because of its low power consumption characteristics. However, conventional 0.9-micron CMOS oscillators are limited to frequencies below 800 MHz. Frequencies as high as 1 GHz are difficult to achieve in 0.9 -micron CMOS, so NMOS is often used for frequencies in that range. Conventional NMOS devices can operate at frequencies that are approximately 30% to 50% higher than those achieved by conventional CMOS. However, NMOS lacks the power-efficiency of CMOS, and it is more expensive to fabricate than CMOS.
Bipolar circuits can provide frequencies above 1 GHz, faster than those provided by conventional NMOS, such as the 1.8 GHz achieved by Nguyen & Meyer and reported in "A 1.8 GHz Monolithic LC Voltage-Controlled Oscillator" IEEE Journal of Solid State Circuits, March, 1992, pp. 444-450. However, the process used for fabricating bipolar circuits is not compatible with MOS fabrication. Furthermore, these devices require more "real estate" on the chip, generate substantially more heat, and lack the low-power performance capability of either type of MOS devices. Therefore they are less suitable than MOS for VLSI structures, generally, and low power applications, in particular.
These higher frequencies, in the multi-gigahertz range, are highly advantageous for multimedia computer workstations, however. Furthermore, they are required by optical communications links and by personal communications systems (PCS) apparatus such as cellular telephones. However, all MOS circuits operate as resistive-capacitive systems which are response-limited by the resulting RC time constant of the circuit.